Conventional tubular fixed bed reactors (TFBR) employed for Fischer-Tropsch (FT) synthesis reactions typically use relatively large catalyst pellets formed as spheres or cylinders with effective diameters of about 2 mm to about 6 mm. With these reactions selectivity to methane tends to increase with increasing diffusion distance into the catalyst. This is believed to be due to the difference in diffusivities between H2 and CO leading to higher H2 to CO ratios near or at the catalytic sites than in the bulk of the reactant fluid. Under Fischer-Tropsch process conditions the pores of the catalyst may be filled with liquid, be this aqueous, organic or a mixture. In addition to this a film of liquid product may cover the surface of the catalyst pellets; this may be product of the reaction or product of the reaction and liquids recycled to the reactor. Liquids fed or formed in the reactor may flow down the length of the reactor under the influence of gravity or the influence of gravity supplemented by the gaseous feed.
Conventionally it is held that the resistance to mass transfer in the pores dominates and this is overcome via either reducing the diffusion length to the catalytic site via reduction in particle diameter or by the use of larger catalyst particles in which the active metals are only deposited in a thin layer of the support close to the particle surface (‘rim’ or ‘eggshell’ type catalysts) thereby reducing the diffusion distance.
The influence of the liquid film formed on the surface of the catalyst particles has largely been ignored as in many cases reactors, both large and small scale, have been operated in regimes in which the influence of the reactor hydrodynamics is not apparent.